Studies of Chemisorption on Metal Loaded Ceria Surfaces

Studies of Chemisorption on Metal Loaded Ceria Surfaces 

Following deposition of an active metal upon a ceria surface, it is possible to study chemisorption on a surface that models many of the important aspects expected for actual ceria supported catalysts. Surface techniques offer the possibility to identify where the adsorbates are located and to identify intermediates that are formed in their interaction. By comparison of ceria surfaces, with and without metal, the synergisms between metal and support can be deduced. By controlled metal deposition, it is possible to study the effects of loading and particle size. By selected preparation of the ceria substrate it is possible to vary factors which may affect the interaction between the metal particle and the ceria, sueh as structure, defeet concentration or oxidation state of the ceria. The goal of chemisorption studies, summarized below, is to relate all these factors to the interaction of the model catalyst with particular adsorbates. 

There have been many studies of the adsorption of CO on ceria surfaces with deposited particles of Rh, Pt, Pd. 22, 0,41,72,80,90-94 mentioned above, CO does not adsorb readily on ceria, but chemisorbs strongly on the metal particles. The major conclusions from these studies is that the nature of the ceria support controls the fate of CO adsorbed on the metal, in part through oxygen transfer between the ceria and the metal particle and partly by other interactions between the metal and the support. 

The adsorption of NO on metal loaded eeria has been examined for Rh/  

Pt/° and Pd. ° As known from work on single crystals, NO dissociates to some extent on each of these metals. The amount of dissociation is dependent upon the structure of the metal surface. Gorte considered Pt and Pd particles deposited on rough, polycrystalline ceria films grown by spray pyrolysis. For Pt they found variation in the TPD results (amount of NO uptake and shape of N2 desorption profile) that varied with the size of the Pt particles. However, the results were comparable to NO TPD results from Pt grown on sapphire. It was concluded that no unusual interaction existed between Pt and the (oxidized) ceria. For Pd it was found that a pronounced difference in the TPD product ratio, NO/N2, occurred for Pd on ceria compared to Pd on sapphire. They attributed the difference to NO adsorption on reduced ceria.  

---